Q: How do I burn a Blu-ray using pre-existing AC-3 audio file?

I'm just wondering what the best settings / option are for compressing the video and burning a Blu-ray from a file exported from FCPX as a master file that used a pre-existing AC-3 audio file. My sound mixer supplied the 5.1 ac-3 audio file. I brought that file into FCPX, set the settings for surround, and it outputted an mov file with 6 channels. As it should be. I still need to compress the video for Blu-ray, but the audio is already compressed and ac-3. Can I compress and burn a Blu-ray through Compressor without recompressing the ac-3 file? Do I set the audio to Target System "Generic AC-3" or do I simply not drop in any audio compression settings at all and only the H.264 for video?

Thanks in advance!